1. Field of the Invention
This invention relates to processes for hydrolyzing starches and more particularly, to such processes especially adapted to provide substrate sugars for the fermentation of ethanol.
2. Description of the Prior Art
With the ever-increasing depletion of economically recoverable petroleum reserves, the production of ethanol from vegetative sources as a partial or complete replacement for conventional fossil-based liquid fuels becomes more attractive. In some areas, the economic and technical feasibility of using a 90% unleaded gasoline-10% anhydrous ethanol blend ("gasohol") has shown encouraging results. According to a recent study, gasohol powered automobiles have averaged a 5% reduction in fuel compared to unleaded gasoline powered vehicles and have emitted one-third less carbon monoxide than the latter. In addition to offering promise as a practical and efficient fuel, biomass-derived ethanol in large quantities and at a competitive price has the potential in some areas for replacing certain petroleum-based chemical feedstocks. Thus, for example, ethanol can be catalytically dehydrated to ethylene, one of the most important of all chemical raw materials both in terms of quantity and versatility.
The various operations in processes for obtaining ethanol from such recurring sources as cellulose, cane sugar, amylaceous grains and tubers, e.g., the separation of starch granules from non-carbohydrate plant matter and other extraneous substances, the chemical and/or enzymatic hydrolysis of starch to fermentable sugar (liquefaction and saccharification), the fermentation of sugar to a dilute solution of ethanol ("beer") and the recovery of anhydrous ethanol by distillation, have been modified in numerous ways to achieve improvements in product yield, production rates and so forth. For ethanol to realize its vast potential as a partial or total substitute for petroleum fuels or as a substitute chemical feedstock, it is necessary that the manufacturing process be as efficient as possible so as to maximize the amount of ethanol produced and enhance the standing of the ethanol as an economically viable replacement for petroleum based raw materials. To date, however, relatively little concern has been given to optimizing the large scale manufacture of ethanol form biomass.
The substitution of alcohol for at least a portion of petroleum based fuels is particularly critical for developing economies where proven domestic petroleum reserves are limited, such as in India and Brazil, and these nations have therefore increasingly emphasized the production of alcohol from vegatative sources, especially from starch-containing grains and tubers.
To make starch an economically attractive source of ethanol, it is essential to achieve rapid and high levels of conversion of the starch to fermentable saccharide and of the fermentable saccharide to ethanol while minimizing consumption of raw materials.
Processes for the acid hydrolysis of starch to provide fermentable saccharides are known (viz., the acid starch hydrolysis processes described in U.S. Pat. Nos. 2,203,325; 2,210,659; 2,359,763; 2,393,095; 2,395,907; 2,565,404; 2,946,706; 2,954,304; 2,989,425; 3,169,083; 3,200,012; 3,236,687; 3,313,654; 3,446,664; 3,484,287; 3,607,395; and, 4,137,094). It is also known from U.S. Pat. No. 2,529,131 to subject still bottoms, or "vinasse", containing unfermented sugars to acid hydrolysis to convert said sugars to fermentable substrate. In all of the afore-said acid hydrolysis processes, prior to conversion of the aqueous fermentable hydrolysate to ethanol employing yeast, the hydrolyzing acid must be neutralized with base. This results in the presence of relatively substantial quantities of salt in the fermentation feed, a condition which is decidedly disadvantageous for optimum ethanol production. And since the salts serve no useful function for any of the discrete operations involved in the conversion of starch to ethanol, their presence as by-products in the sugar liquor merely represents a wasteful consumption of raw materials, i.e., acid and neutralizing base.
Processes for the enzymatic hydrolysis of starch to provide fermentable saccharides are also well known (viz., U.S. Pat. Nos. 2,219,668; 2,289,808; 2,356,218; 2,431,004; 2,676,905; 2,954,304; 3,308,037; 3,337,414; 3,423,239; 3,425,909; 3,551,293; 3,565,764; 3,592,734; 3,654,081; 3,720,583; 3,910,820; 3,912,590; 3,922,196; 3,922,197; 3,922,198; 3,922,199; 3,922,200; 3,922,201; 3,969,538; 3,988,204; 3,922,261; 3,966,107; 3,969,538; 3,998,696; 4,014,743; 4,016,038; 4,017,363; 4,028,186; and 4,032,403; see also, Novo Industri A/S (DK-2880 Bagsvaerd, Denmark) brochures entitled "Dextrose and Starch Sugar", "Conversion of Starch" and "Glucose Syrup").
While the enzyme hydrolysis of starch generally provides sugar which is of a superior quality compared to that obtained by acid hydrolysis, the somewhat longer conversion times and the relatively high cost of enzymes tend to militate against the use of enzyme hydrolysis methods where maximum rates and levels of sugar production, not product quality, is of foremost consideration.
Accordingly, there has heretofore existed a need for a process for hydrolyzing an aqueous starch slurry to a solution of fermentable sugar starch at rapid and high levels of conversion while dispensing with or minimizing the need to neutralize acid present in the sugar solution.